Some users do not receive the invite email that we manually send. We can only send it once, so we send a general login link instead, but when they try to login, they receive a message that their email does not have access to the app, even though that email is in the Users table.
This happens about 10-20% of the time for new users.
Are you on a Free or Maker plan? Are your users trying to sign in with business email addresses? Those plans only allow for personal users using personal email addresses.